Food, pharmaceutical, and industrial films and coatings contain a polymeric base that often is supplemented with plasticizers, detacifiers, surfactants, and coloring agents. Typically used polymers include gums; cellulose derivatives or hydrolysis products; synthetic polymers such as polyvinyl alcohol, polyvinyl acetate, polyurethane, polystyrene or polyvinylpyrrolidone; gelatin; dextrins; modified cook-up-starches, and combinations of the foregoing. These polymers are often very expensive or difficult to use, or have reduced acceptance by certain segments of the consuming public. In recent years, greater emphasis has been placed on replacing all or part of these polymer systems with more economical consumer-friendly starch-based polymers.
Many starch materials have been used to make a variety of films, foams, and other industrial and food products. However, despite the variety of starch materials available, known starches generally can be somewhat unsuitable for use in these applications. For instance, native starches have two key limitations when used in films and coatings. Films made from unmodified or “reduced viscosity” starches generally are brittle, weak, cloudy, and opaque, and cooking is generally required to hydrate the starch polymers, inasmuch as native starches typically are water insoluble at temperatures at or below room temperature (25° C.). The problems of brittleness, clouding and opacity can be mitigated somewhat with a low degree of hydroxyalkylation of amylase and/or amylopectin contained in the starch to form a hydroxyalkyl starch, but still the hydroxyalkyl starch will be cold-water insoluble. Thus, such starches are not useful where heating is not available.
To overcome the problem of cold-water insolubility, the starch may be physically or chemically modified, or may be enzymatically treated. One approach known in the art is to modify the starch by using alkylene oxide reagents, such as propylene, oxide, ethylene oxide, and the like. This process generally requires the use of organic solvents, such as ethanol, which are undesired due to the additional processing costs associated with such solvents.
The prior art also has taught to hydroxyalkylate the starch using an aqueous process. The hydroxyalkyl starch thus prepared is then cooked by drum-drying or spray-drying, and is ground to be marketed as a pre-gelled or “instant” starch. While such pre-gelled starches are suitable for some applications, such starches are difficult to disperse in water in low temperatures. Starches used in film and coating applications may contain intact starch granules, which can result in poor film clarity and increased film opacity. Particularly in the case of drum-dried starches, large lumps, sometimes referred to as “fish-eyes,” are often formed. Also, the viscosity of these starches often is high, thus limiting the level of solids, which can be dispersed in an aqueous system without resulting in mixing and handling problems. Moreover, while occasionally additives such as borax, boric acid, gum arabic, and sulfate salts are added to improve wettability or dispersability, these solutions are somewhat unsatisfactory because of the additional costs required for such additional ingredients.
Attempts also have been made to formulate a pre-gelled, starch using an extruder. However, such attempts often have resulted in processing difficulties, particularly when modified starches are extruded under conditions of low moisture. For example, U.S. Pat. No. 5,849,233 discloses a method of extruding starch. This reference recognizes processing difficulties in extruding starches, and purports to teach that these difficulties can be overcome by employing as a feed starch a starch with a coarse particle size. However, the process requires additional drying and conditioning equipment, and can entail extra processing costs. Other efforts to extrude starch (e.g., as shown in International Publication WO 00/08945, U.S. Pat. No. 3,904,429 and Canadian patent, 1,286,533) have not provided a cold water soluble starch that is film-forming in aqueous solution. The invention seeks to address these shortcomings in the art.